A 57 is a low output dynamic mic mostly used for loud sources like amp cabs and drums. You need a good preamp to boost the signal for something relatively quiet like an acoustic guitar. I would try plugging your guitar into an amp and hanging the 57 in front of it or pick up a condenser mic and use phantom power. Condensers output something like 10 times the signal a dynamic mic does. Did a quick search about this and your Focusrite and found this on Gearslutz:

The issue of insufficient Scarlett 2i2 mic pre gain using a SM57, SM58 or SM7B comes up at least once a month on this forum, so a search of posts will turn up the same information and advice. It has not changed in years since the 2i2 was released.

Having stated all of the above, the actual mic input gain of the Scarlett 2i2 using the XLR input is very low for use with a Shure SM57. The SM57 output, for a quite loud sound pressure level of 94 dB SPL, is only -56dBV.

That means that is someone is speaking into a SM57 at a relatively loud sound level of 94dB SPL, and the mic is connected to a Scarlett 2i2 Mic input set to maximum gain, the digital recording level will not exceed -10dB(FS). Also, the Scarlett 2i2 Mic pres are fairly noisy when operated at full gain.

They can work acceptable with with a high-output, phantom-powered condenser mic, but really don't have enough gain for a low-output dynamic like a SM57 or SM58, unless you are only recording very loud sounds. A loud drum set or a close-miced guitar amp producing a SPL of 105-110 dB SPL is fine, but many other potential sound sources are simply not loud enough.

This is a detailed technical explanation of what I just wrote. You need a higher source signal or a condenser mic or a better preamp.

Bob

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